In magnetic tape digital data recording systems, the general practice is to record the data in blocks which are separated by empty interblock gaps. The data blocks are recorded or reproduced at a substantially constant tape speed. The gaps provide the space and time for stopping the tape when the system is not actively recording or reproducing data, and for subsequently starting the tape and bringing it up to normal operating speed before recording or reproducing the next data block.
The required interblock gap length depends on the performance level of the transport, the tape operating speed and, sometimes, industry, national or international standards. High tape speeds and/or less sophisticated transports require longer gaps. Shorter gaps require lower operating tape speeds and/or higher performance transports. The general range of interblock gaps is from about 0.5" to several inches, with about 1.5" being a common value.
The feasible data recording densities have increased from a few hundred bpi (bits per inch) a few decades ago to 6400 bpi and higher now. However, when the tape space which is lost to the interblock gaps is considered, the effective data recording density has generally not increased nearly as much as the block recording density.
For example, consider the recording of data in bit serial form in 4096 bit blocks. If the block recording density is 6400 bpi,the data block length will be 4096/6400=0.64". However, if the interblock gap length is 1.5", the effective total block length will be 2.14". The effective recording density is then only 4096/2.14=1914 bpi, less than a third of the 6400 bpi block density. It is this 1914 bpi effective recording density, not the 6400 bpi block density, that determines how much data can be recorded on a reel or cartridge of tape.
The effective recording density, D.sub.eff, in bpi, is given by: ##EQU1## where: B=the data block length in bits; D=the data block recording density in bpi; and
G=the interblock gap length in inches. PA1 S=the stopping distance in inches; PA1 V=the tape speed in in/sec; and PA1 g=the acceleration of gravity, 385.8 in/sec.sup.2.
Values of D.sub.eff for various values of B, D and G are listed in Table 1. As may be seen from Table 1, short block lengths and long gap lengths give low effective recording densities, regardless of the block recording densities. As would be expected, the relative loss of recording density is greater for higher block recording densities. To obtain the benefits of a high block recording density, in the form of a high effective recording density, it is necessary to use long data block lengths and/or short interblock gaps.
The use of long data blocks can have a number of cost and/or performance disadvantages. It is generally necessary that the recording/reproducing system have a buffer memory which is large enough to store one or more complete data blocks. This may be a significant cost. In some applications, the buffer memory functions may be provided by the memory of an associated computer. However, in such instances, the computer must devote a significant amount of memory to the recording/reproducing task whenever it is being performed. Further, since the tape system is then not an independent unit, but must make substantial use of the computer for its basic operation, system integration may be more difficult and may require individual engineering and/or programming for the specific computer system. If a computer memory is used for the recorder buffer memory, recorder processes such as redundant encoding, error detection and error correction may become computer system tasks, rather than being performed within an independent recorder.
There are also performance disadvantages to the use of long data blocks. With fixed length blocks and variable length files, longer blocks may increase the probable waste of recording space by the incomplete filling of a block. The longer the data block, the more likely it is that there will be an error in reading the block. If there is a read error, and the system re-reads a data block, it will require more time to re-read a longer block.
______________________________________ D, Data E.sub.ff, Effective Recording Block B, Data Data Density, in bpi, for Recording Block Block Specified Interblock Density Length Length Gap Length, G in bpi in Bits in Inches 1.5" .5" .25" .1" ______________________________________ `200 256 1.28 92 144 167 186 1,024 5.12 155 182 191 196 4,096 20.48 186 195 198 199 16,384 81.92 196 199 199 200 800 256 .32 141 312 449 610 1,024 1.28 368 575 669 742 4,096 5.12 619 729 763 785 16,384 20.48 745 781 790 796 1600 256 .16 154 388 624 985 1,024 .64 479 898 1,151 1,384 4,096 2.56 1,009 1,339 1,458 1,540 16,384 10.24 1,396 1,526 1,562 1,585 6400 256 .04 166 474 883 1,829 1,024 .16 617 1,552 2,498 3,938 4,096 .64 1,914 3,593 4,602 5,535 16,384 2.56 4,035 5,354 5,831 6,159 ______________________________________
A second approach to increasing the effective data recording density is to reduce the interblock gap length. A problem with a short gap is that it requires high accelerations to stop the tape within a short gap, and to start the tape and bring it up to normal operating speed within a short gap. High tape accelerations generally require more expensive transport mechanisms, and may increase tape wear and the likelihood of tape damage.
If we consider the stopping of the tape within a gap, and assume that the tape deceleration is constant until the tape stops, the acceleration is given by: EQU Acceleration (in/sec.sup.2)=V.sup.2 /2S
or: EQU Acceleration (grav.)=V.sup.2 /2Sg
where:
The same magnitude of acceleration is required to start the tape and bring it up to a normal operating speed, V, within a distance, S.
Table 2 lists calculated tape acceleration values for various values of V, the tape velocity; S, the tape stopping distance and G, the interblock gap length. The stopping distance, S, is assumed to be one-third of the interblock gap length. As may be seen, short interblock gaps imply high tape accelerations.
The general problem is that high data block recording densities do not necessarily give correspondingly high effective recording densities. In order to achieve high effective recording densities, it is generally necessary to use long data blocks and/or short interblock gaps. In a general purpose magnetic tape digital data recording system, it is desirable to avoid a requirement of long blocks. Short interblock gaps, with reasonable tape speeds, imply high tape stopping and starting accelerations. Reducing the tape speed is generally an undesirable solution as it reduces the recording/reproducing data transfer rate. In a typical system, the feasible tape acceleration sets a lower limit on the interblock gap length and, thus, limits the effective recording density.
______________________________________ Tape Acceleration in in/sec.sup.2 for Specified Interblock Gap, G, and Stopping Distance, S Tape Speed G = 1.5" G = .5" G = .25" G = .1" V in in/sec S = .5" S = .167" S = .0833" S = .0333" ______________________________________ 7.5 56 168 338 845 30 900 2,695 5,402 13,514 75 5,625 16,841 33,764 84,459 ______________________________________ Table 2, tape acceleration in in/sec.sup.2 required to stop or start the tape in a conventional tape transport system, as a function of: V, the tape speed in in/sec; and S, the tape stopping or starting distance in inches. The stopping distance, S, is assumed to be onethird of the interblock gap length, G.